Many conventional applications of therapeutic ultrasound have employed low frequency transducers. These transducers have operational frequencies that typically range from 500 kHz to 1.5 MHz. Such low frequency transducers are often preferred because they allow for acoustical energy to be focused deep into the body, without harming the overlying tissue structures.
A conventional application of non-invasive therapeutic ultrasound using a low frequency transducer is depicted in FIG. 1. A conventional therapeutic system 100 comprises a transducer 102 that uses low frequency energy to treat a deep treatment region 110. Deep treatment region 110 is located at a deep depth 106 below a superficial region 112, e.g., tissue layers and structures, and a subcutaneous region 114 of a patient. Deep depth 106 may range from several millimeters to 5-7 centimeters or more. Conventional system 100 cannot treat superficial regions 112 or subcutaneous regions 114 through use of low-frequency transducer 102, thus limiting the applications of such systems. For example, some cosmetic surgeries may also need to provide treatment to superficial and/or subcutaneous, as well as deep treatment regions, thus eliminating the use of lower frequency transducers.
Another undesirable side effect of low-frequency therapy is that the acoustic energy must pass through intervening tissue layers before reaching the desired deep treatment area. The intervening layers tend to defocus the rays and absorb some of the acoustic energy. This causes the focal spot size to widen, making it difficult to control the location of the focal spot.